The prosthetic replacement of joints, such as the shoulder, hip, knee, ankle, and wrist has evolved over the years from early, relatively crude models to current prostheses which closely duplicate functions and motions of a natural joint. As a result, prosthetic joints have provided patients with increasing comfort, freedom of motion and an ability to lead nearly normal lives.
Although there have been problems with excessive wear between components of prosthetic devices which move with respect to each other, by and large the fixation of the prosthetic components to the patient's bone structure did and continues to represent the greatest difficulty. Early attempts to solve this problem by use of what were thought to be mechanical locks between the implants and the bone and by use of tissue compatible acrylic cement were discussed in the inventor's copending application, Ser. No. 590,258 filed Mar. 16, 1984.
In cemented implants the cement initially acts as a grout to "form-fit" the implant to the bone. It then "cold-cures" to a hard material which mechanically fixes itself to the bone by interdigitating into the bone trabeculae. This ability of the cement to fix a metal or plastic implant component securely to bone is the main factor which has so greatly improved the status of joint replacement over the prior state of merely implanting the component into bone and hoping that it actually stayed securely in place. Most of the components implanted without cement were not securely fixed and ultimately came loose in the bone with subsequent pain and failure of the procedure. Thus, cement fixation of implants gives excellent short-term results; however, in younger, heavier, or more active individuals, the bond between bone and cement eventually broke down. The result was a loosening or separation between the cement-bone-implant interface which placed the device back into a category similar to implanted devices prior to use of cement-fixation, i.e., the implant was not securely fixed to the bone and pain which indicated a failure of the procedure was the end-result.
As a consequence of increasing numbers of failures with cemented devices, alternative methods of fixation of the implant were sought. One alternative, which is currently the state of the art for implant fixation, is to coat the surface of the implant with a porous material to allow the patient's bone to grow into the pores, thereby biologically fixing the implant to the bone. This appears to be the ideal method of implant fixation. The patient's own tissue now holds the implant and the latter has become a permanent part of the bone, thus obviating the problem of implant loosening.
Another problem encountered with joint implants is an abnormal stress transference from the implant to the bone. The ideal stress transference of load to the bone is the normal, anatomical transference. To approximate it, the implant material should have mechanical properties similar to those of the bone and should replace only the destroyed joint surface. This would place no implant material, or only a minimal amount of implant material, within (intramedullary) the bone. This is most difficult to do with joints having porous implant surfaces because they require immediate rigid fixation for a sufficient time period to assure at least six to twelve weeks in growth time. If the device is not held rigidly, there will be micro-motion occurring at the implant-bone surface which results in a fibrous tissue in-growth rather than the necessary securely-fixed bony in-growth. The currently most common method of holding the implant rigidly in the bone is by providing the implant with a stem which "press-fits" into the intramedullary cavity of the bone, e.g., the femur, or if no such cavity is present, by anchoring the implant to the bone, e.g., the pelvis, with a threaded anchor bolt. Such a press-fit of the stem into the shaft of the bone holds the device rigidly and allows an adequate bone in-growth for secure fixation. For the surgeon it also provides the desired proper anatomical placement of the implant in the bone in a reproducible manner.
The short coming of the aforesaid approach is that loading of the bone is no longer physiologic. Instead of being loaded primarily at the end of the bone near the joint surface as in the normal situation, the bone becomes loaded more distally in the shaft where the stem of the implant is fixed to the bone. The result is an abnormal transference of stress which bypasses or "unloads" the end or joint surface portion of the bone, with a subsequent resorption of that bone. This leads to a weakening of bone in that area over a period of years, thus creating the potential for fracture or disappearance of the bone that previously held the implant securely. The result is again a loosening of the implant within the bone with all the adverse consequences previously mentioned.
For implants, such as an acetabular cup of a hip prosthesis, which are vertically held in place with a screw, the non-physiologic transference of stresses is less pronounced because the location and orientation of the anchoring bolt can be selected to minimize non-natural load transference stresses. Nevertheless, at the very least the presence of the anchoring bolt in the bone weakens the latter and is undesirable for that reason alone.
Thus, if a stem placed down the medullary cavity of the bone (for a correct alignment of the implant and for its immediate, rigid fixation to allow bony in-growth fixation) produces an abnormal stress distribution, it would appear obvious to utilize an implant without a stem. In such a case the implant would essentially only resurface the destroyed articular surface. This is more readily done in joints, such as the knee, elbow, or ankle, than in others, such as the hip, shoulder or wrist. However, even if a stemless implant is feasible, its immediate rigid fixation is not as secure as if the implant were anchored with a stem, or an anchor bolt. When the stem functions to correctly align an anchor or an anchor bolt in its correct position until bone in-growth is complete, an alternative mechanism is necessary therefore to accomplish the functions of the stem. One such mechanism could be a transcortical fixation of the implant such as multiple screws. This, however, makes it more difficult for the surgeon to correctly and reproducibly position and align the implant.